1. Field of the Invention
This invention relates to a method for excavation from the top down, usually known as "undercut". More particularly the invention relates to an undercut excavation method using posts which are adapted to support concrete floors that become a roof for the next lower cut or excavation level.
2. Discussion of the Prior Art
The excavation method of the present invention is particularly well suited to excavation of material having poor structural cohesion, such as overburden tills below proposed highrise buildings or of badly fractured or unstable mine rock. The so called "undercut-and-fill" mining method is especially well adapted for the purposes of the present invention. There are many descriptions of the conventional undercut-and-fill mining method in the mining literature, however, probably one of the best is to be found in the article entitled: "Undercut-and-Fill Mining at the Frood-Stobie Mine of the International Nickel Company of Canada, Limited" by J. A. Pigott and R. J. Hall published in The Canadian Mining and Metallurgical Bulletin for June, 1961, Montreal, pp. 420-424.
It is also already known to mine ore by an undercut-and-fill method while providing concrete floors that serve as a roof for the subsequent cut on a lower level. For example, in an article entitled "Kosaka Mine and Smelter" published in the Mining Magazine--November 1984, page 404, a method called underhand cut and fill using an "artificial roof" is disclosed. According to this method, the cross-cuts are back-filled by first installing a layer of reinforcing steel mesh near the floor, followed by pumping in a 500-600 mm thickness of a comparatively weak concrete mix and, when it is dry, backfilling with a mixture of sand, volcano ash and 3.5% cement. When alternate cross-cuts have been completed across the length of the mining block, the intermediate 4 meter wide ribs of ore are also extracted, so that the entire slice of ore is replaced by a continuous layer of reinforced concrete topped by loosely cemented fill. Then, when mining of the next lower cut is undertaken, the concrete which has been placed on the floor of the level above, now forms an artificial roof. However, because of such ground conditions, timber sets are installed at 1 meter intervals under such artificial roof to support the same when excavating the lower cut.
The main problem with the above method is that when mining is carried out under the artificial concrete roof, initially there is no support provided for this roof, and until such support is provided by means of timber sets that are needed at intervals as close as 1 meter apart, workers are exposed to safety hazards from potential fall of the roof and of materials above such roof during the temporary periods of unsupport. Another problem is the requirement of providing supporting timber sets at 1 meter intervals. Due to this, the excavated work area becomes congested with supports, thus restricting the excavation rate to small equipment with limited movement, at high unit cost. Also, short ramps (two meters or less) are required to prevent damage to posts and to limit the unsupported spans.
The cost component is an important consideration in mining operations and can dictate the economic viability of several known ore bodies which are presently considered for mining by the undercut-and-fill method. The novel method of the present invention, which lends itself to an efficient, high productivity mechanized excavation will be particularly suitable for such ore bodies.
In the area of civil engineering, the excavation from the top down is presently carried out with the use of sheet piles at great cost. The method of the present invention will again provide a relatively inexpensive and entirely viable replacement for such known practice.